Automatable scan partitioning for low power using external control

ABSTRACT

Scan architectures are commonly used to test digital circuitry in integrated circuits. The present invention describes a method of adapting conventional scan architectures into a low power scan architecture. The low power scan architecture maintains the test time of conventional scan architectures, while requiring significantly less operational power than conventional scan architectures. The low power scan architecture is advantageous to IC/die manufacturers since it allows a larger number of circuits (such as DSP or CPU core circuits) embedded in an IC/die to be tested in parallel without consuming too much power within the IC/die. Since the low power scan architecture reduces test power consumption, it is possible to simultaneously test more die on a wafer than previously possible using conventional scan architectures. This allows wafer test times to be reduced which reduces the manufacturing cost of each die on the wafer.

[0001] This disclosure TI-32158 relates to and incorporates by referenceTI patent application Ser. No. 60/187,972 (TI30725), filed Mar. 9, 2000,Adapting Low Power Scan Architectures for Low Power Operation.

PRIOR ART DESCRIPTION

[0002]FIG. 1 illustrates a conventional scan architecture that a circuit100 can be configured into during test. In the normal functionalconfiguration, circuit 100 may be a functional circuit within IC, but intest configuration it appears as shown in FIG. 1. Scan architectures canbe applied at various circuit levels. For example, the scan architectureof FIG. 1 may represent the testing of a complete IC, or it mayrepresent the testing of an embedded intellectual property coresub-circuit within an IC, such as a DSP or CPU core sub-circuit. Thescan architecture includes a scan path circuit 104, logic circuitry tobe tested 108, and connection paths 112-120 to a tester 110. Tester 110operates to; (1) output control to operate scan path 104 via controlpath 114, (2) output serial test stimulus patterns to scan path 104 viascan input path 118, (3) input serial test response patterns from scanpath 104 via scan output path 120, (4) output parallel test stimuluspatterns to logic 108 via primary input path 112, and (5) input paralleltest response patterns from logic 108 via primary output path 116. Scanpath 104 operates, in addition to its scan input and scan output modesto tester 110, to output parallel test stimulus patterns to logic 108path 122, and input parallel response patterns from logic 108 via path124.

[0003] Typically tester 110 is interfaced to the scan architecture byprobing the die pads at wafer level, or by contacting package pins afterthe die is assembled into a package. While tester 110 connections to theprimary inputs 112 and primary outputs 116 of logic 108 are shown, theprimary input and output connections could be achieved by augmentationof scan path 104. For example, scan path 104 could be lengthened toinclude boundary scan cells located on each primary input and primaryoutput of logic 108. The boundary scan cells would provide primaryinputs to and primary outputs from logic 108, via widened stimulus andresponse busses 122 and 124, respectively. In some instances, logic 108may be sufficiently tested by scan path 104 such that it is notnecessary to provide primary inputs to and outputs from logic 108 viathe tester or via the above described augmentation of scan path 104. Forexample, if the amount of logic 108 circuitry made testable by the useof scan path 104 in combination with the primary inputs and outputs isvery small compared to the amount of logic 108 circuitry made testableby the scan path 104 alone, then the primary input and outputconnections to logic 108 may removed without significantly effecting thetest of logic circuitry 108. To simplify the description of the priorart and following description of the present invention, it will beassumed that logic circuit 108 is sufficiently tested using only scanpath 104, i.e. the primary inputs 112 and primary outputs 116 are notrequired. However, it is clear that primary input and output connectionsto the tester or to an augmented scan path 104, as described above,could be used as well.

[0004]FIG. 2 illustrates an example of a conventional scan cell thatcould be used in scan path 104. (Note: The optional scan cellmultiplexer 218 and connection paths 220 and 224, shown in dotted line,will not be discussed at this time, but will be discussed later inregard to FIGS. 7 and 8.) The scan cell consists of a D-FF 204 and amultiplexer 202. During normal configuration of the circuit 100,multiplexer 202 and D-FF 204 receive control inputs SCANENA 210 andSCANCK 212 to input and output functional data to logic 108 via paths206 and 216, respectively. In the normal configuration, the SCANCK toD-FF 204 is typically a functional clock, and the SCANENA signal is setsuch that the D-FF always clocks in functional data from logic 108 viapath 206. During the test configuration of FIG. 2, multiplexer 202 andD-FF 204 receive control inputs SCANENA 210 and SCANCK 212 to capturetest response data from logic 108 via path 206, shift data from scaninput path 208 to scan output path 214, and apply test stimulus data tologic 108 via path 216. In the test configuration, the SCANCK to D-FF204 is the test clock and the SCANENA signal is operated to allowcapturing of response data from logic 108 and shifting of data from scaninput 208 to scan output 214. During test configuration, SCANENA iscontrolled by tester 110 via path 114. SCANCK may also be controlled bythe tester, or it may be controlled by another source, for example afunctional clock source. For the purpose of simplifying the operationaldescription, it will be assumed that the SCANCK is controlled by thetester.

[0005] The scan inputs 208 and scan outputs 214 of multiple scan cellsare connected to form the serial scan path 104. The stimulus path 216and response path 206 of multiple scan cells in scan path 104 form thestimulus bussing path 122 and response bussing path 124, respectively,between scan path 104 and logic 108. From this scan cell description, itis seen that the D-FF is shared between being used in the normalfunctional configuration and the test configuration. During scanoperations through scan path 104, the stimulus outputs 216 from eachscan cell ripple, since the stimulus 216 path is connected to the scanoutput path 214. This ripple causes all the inputs to logic 108 toactively change state during scan operations. Rippling the inputs tologic 108 causes power to be consumed by the interconnect and gatingcapacitance in logic 108.

[0006]FIG. 3 illustrates a simplified example of how tester 110 operates300 the scan architecture during test. Initially the tester will outputcontrol on path 114 to place the scan architecture in an idle state 302.Next, the tester outputs control on path 114 to place the scanarchitecture in an operate scan path state 304. In the operate scan pathstate, the tester outputs control to cause the scan path to acceptstimulus data from the tester via path 118 and to output response datato the tester via path 120. The tester maintains the operate scan pathstate until the scan path has been filled with stimulus data and emptiedof response data. From the operate scan path state, the tester outputscontrol on path 114 to place the scan architecture in a capture responsedata state 306. In the capture response data state, the tester outputscontrol to cause the scan path to load response data from logic 108 viapath 124. From the capture response data state, the tester outputscontrol on path 114 to cause the scan architecture to re-enter theoperate scan path state. The process of entering the operate scan pathstate 304 to load stimulus into the scan path and empty response fromthe scan path, then passing through the capture response state 306 toload new response data from logic 108 repeats until the end of test. Atthe end of test the tester outputs control to cause the scanarchitecture to re-enter the idle state 302.

[0007]FIG. 4 illustrates a timing example of how tester 110 outputsSCANENA and SCANCK signals to scan path 104 during scan operations. Inthis example, a high to low transition on SCANENA, at time 406, incombination with SCANCKs occurring during time interval 402, causesstimulus data from the tester to be input to the scan path via path 118while response data is output from the scan path to the tester via path120. A low to high transition on SCANENA, at time 408, in combinationwith a SCANCK at time 404, causes response data from logic 108 to beloaded into the scan path. Time interval 402 relates to operate scanpath state 304 and time interval 404 relates to capture response state306 of FIG. 3. As seen in the timing and operation diagrams of FIGS. 3and 4, the time interval sequences 404 (i.e. state 306) and 402 (i.e.state 304) cycle a sufficient number of times during test to input allstimulus to and obtain all response from logic 108.

[0008] From the scan architecture described in regard to FIGS. 1, 2, 3,and 4 it is seen that the stimulus 122 outputs ripple the inputs tologic 108 as data shifts through the scan path 104 during scanoperations. Rippling the inputs of logic 108 causes simultaneouscharging and discharging of capacitance's associated with theinterconnects and gates of logic 108. For example, each scan cellstimulus output 216 to logic 108 charges and discharges a certain amountof capacitance within logic 108 at a frequency related to the data bitsbeing scanned through the scan cell. While each scan cell stimulusoutput may only be directly input to a few gates within logic 108, eachof the gates have outputs that fanout to inputs of other gates, and theoutputs of the other gates again fanout to inputs of still furthergates, and so on. Thus a transition on the stimulus output of a singlescan cell may initiate hundreds of transitions within logic 108 as aresult of the above mentioned signal transition fanout. Each of thetransitions charge or discharge a portion of the total capacitance withlogic 108 and therefore contribute to power consumption within logic108.

[0009] The individual power (Pi) consumed by the rippling of a givenscan cell output 216 can be approximated by CV²F, where C is thecapacitance being charged or discharged by the scan cell output (i.e.the capacitance of the above mentioned signal transition fanout), V isthe switching voltage level, and F is the switching frequency of thescan cell output. The total power (Pt) consumed by simultaneouslyscanning all the scan cells in scan path 104 is approximately the sum ofthe individual scan cell powers, i.e. Pt=Pi₁+Pi₂ +. . . Pi_(N). Thetotal power consumed by circuit 100 when it is configured into the scanarchitecture of FIG. 1 can exceed the power consumed by circuit 100 whenit is configured into its normal functional mode. This can be understoodfrom the fact that, during normal functional mode of circuit 100, notall the D-FFs 204 simultaneously operate, as they do during scanoperations occurring during the above described scan test operation.Further if an IC contained multiple circuits 100, the test of the IC mayrequire testing each circuit 100 individually due to the above describedtest power consumption restriction. This lengthens the test time of theIC, which increases the cost to manufacture the IC.

[0010] A first known method of reducing power consumption during testoperation is to insert blocking circuitry, such as a gate, into thestimulus paths 216 of each scan cell, such that during scan operationsthe inputs to logic 108 are blocked from the effect of the scan ripple.The problem with the first method is that it adds an undesirable delay(i.e. the blocking circuit delay) in the stimulus paths 216 betweenD-FFs 204 and logic 108. This delay can negatively effect theperformance of circuit 100 when it is configured into its normalfunctional mode. A second known method is to reduce the scan clock rate,such that the ripple frequency (F) is reduced. The problem with thesecond method is that it increases the test time since scan operationsare performed at the reduced scan clock rate.

[0011] Today, there are a number of test synthesis vendor tools that cansynthesize and insert scan architectures into ICs, similar in structureto the scan architecture shown in FIG. 1. The use of such “push-button”scan insertion tools is an attractive alternative to customized scandesigns since it is an automated process. As will be described, thepresent invention provides a method of adapting these synthesized scanarchitectures such that they may operate in a desired low power mode.The process of adapting scan architectures for low power operation isalso easily automated.

INVENTION DESCRIPTION

[0012] The present invention described below provides a method ofadapting synthesized scan architectures to achieve a low power mode ofoperation. The process of adapting scan architectures for low poweroperation is achieved without the aforementioned problems of; (1) havingto insert blocking circuitry in the stimulus paths which adds signaldelays, and (2) having to decrease the scan clock rate which increasestest time. Furthermore, as will be described in more detail later, theprocess of adapting scan architectures for low power operation isachieved without having to modify the stimulus and response testpatterns which are automatically produced by scan architecture synthesistools.

[0013] The circuits and processes disclosed in this patent are used inmanufacturing to test and ensure proper operation of the integratedcircuit products before sale. The circuits and processes disclosed inthis patent can also be used after the sale of the integrated circuitproducts to test and ensure the continued proper operation of theintegrated circuit products and possibly to develop and test softwareproducts associated with the integrated circuit products.

[0014]FIG. 5 illustrates the scan architecture of FIG. 1 after it hasbeen adapted into the low power scan architecture of the presentinvention. The changes between the FIG. 1 scan architecture and the FIG.5 low power scan architecture involve modification of scan path 104 intoscan path 502, and the insertion of an adaptor circuit 504 in thecontrol path 114 between tester 110 and scan path 502.

[0015] Adapting scan path 104 into scan path 502 involves reorganizingscan path 104 from being a single scan path containing all the scancells (M), into a scan path having a desired number of selectableseparate scan paths. In FIG. 5, scan path 502 is shown after having beenreorganized into three separate scan paths A, B, and C 506-510. It isassumed at this point in the description that the number of scan cells(M) in scan path 104 is divisible by three such that each of the threeseparate scan paths A, B, and C contains an equal number of scan cells(M/3). The case where scan path 104 contains a number of scan cells (M)which, when divided by the number of desired separate scan paths, doesnot produce an equal number of scan cells in each separate scan pathwill be discussed later in regard to FIG. 9.

[0016] Scan paths A, B, and C are configured as follows inside scan path502. The serial input of each scan path A, B, and C is commonlyconnected to tester 110 via connection path 118. The serial output ofscan path A is connected to the input of a 3-state buffer 512, theserial output of scan path B is connected to the input of a 3-statebuffer 514, and the serial output of scan path C is connected to theinput of a 3-state buffer 516. The outputs of the 3-state buffers512-516 are commonly connected to tester 110 via connection path 120.Scan paths A, B, and C each output an equal number of parallel stimulusinputs 526, 530, 534 to logic 108, and each input an equal number ofparallel response outputs 524, 528, 532 from logic 108. The number ofstimulus output signals to logic 108 in FIGS. 1 and 5 is the same. Thenumber of response input signals from logic 108 in FIGS. 1 and 5 is thesame. Scan path A and buffer 512 receive control input from adaptor 504via bus 518, scan path B and buffer 514 receive control input fromadaptor 504 via bus 520, and scan path C and buffer 516 receive controlinput from adaptor 504 via bus 522.

[0017] Adaptor 504 is connected to scan paths A,B,C via busses 518-522and to tester 110 via bus 114. The purpose of the adaptor is tointercept the scan control output 114 from tester 110 and translate itinto a sequence of separate scan control outputs 518-522 to scan pathsA, B, and C, respectively. Each of the separate scan control outputs518-522 are used to operate one of the scan paths A, B, and C.

[0018]FIG. 6 illustrates a simplified example of the combined operation600 of the tester 110 and adaptor 504 during test. The operation oftester 110 is the same as previously described in regard to FIG. 3. Whenthe tester transitions to the operate scan path state 304, it beginsoutputting control to adaptor 504 via path 114. The adaptor responds tothe tester control input by translating it into a sequence of separatecontrol outputs 518, 520, and 522 to scan paths A, B, and C. Asindicated in adaptor operation block 602, the adaptor first responds tocontrol 114 during adaptor operate state 604 to output control 518,which enables buffer 512 and operates scan path A to input stimulus datafrom tester 110 via path 118 and output response data to tester 110 viapath 120. After scan path A is filled with stimulus and emptied ofresponse, adaptor 504 responds to control 114 during operation state 606to output control 520, which enables buffer 514 and operates scan path Bto input stimulus data from tester 110 via path 118 and output responsedata to tester 110 via path 120. After scan path B is filled withstimulus and emptied of response, adaptor 504 responds to control 114during operation state 608 to output control 522, which enables buffer516 and operates scan path C to input stimulus data from tester 110 viapath 118 and output response data to tester 110 via path 120. After scanpaths A, B, and C have been filled and emptied, the tester 110transitions from the operate state 304, through the capture state 306,and back to the operate state 304. During this transition, the adaptoris idle during the capture state 306, but resumes its scan controlsequencing operation when the operate state 304 is re-entered. Thisprocess of sequentially scanning scan paths A, B, and C, then performinga capture operation to load response data repeats until the test hasbeen performed and tester 110 enters the idle state 302.

[0019] During the sequencing of the operation states 604-608, only oneof the buffers 512-516 are enabled at a time to output response data totester 110. Also, the sequencing of the adaptor operation states 604-608occurs in a seamless manner such that the stimulus data from the tester110 is input to scan path 502 as it was input to scan path 104, and theresponse data to tester 110 is output from scan path 502 as it wasoutput from scan path 104. To the tester, the behavior of the scan path502 and adaptor 504 combination is indistinguishable from the behaviorof the scan path 104 in FIG. 1. Thus the test time of the logic 108 inFIG. 5 is the same as the test time of logic 108 in FIG. 1.

[0020] From the above description, it is seen that only a subset (i.e.subset A 526, B 530, or C 534) of the stimulus input bus 122 to logic108 is allowed to ripple at any given time during the adaptor operatedscan operation of FIGS. 5 and 6. In contrast, the entire stimulus inputbus 122 to logic 108 ripples during the tester operated scan operationof FIGS. 1 and 3. Since, using the present invention, only a subset ofthe stimulus inputs to logic 108 are allowed to ripple at any one time,less of the aforementioned interconnect and gating capacitance of logic108 is simultaneously charged and discharged during scan operations. Byreducing the amount of logic 108 capacitance being simultaneouslycharged and discharged during scan operations, the power consumed bylogic 108 is advantageously reduced by the present invention.

Example Adaptor Circuit

[0021]FIG. 7 illustrates an example adaptor circuit 504 implementation.Adaptor 504 inputs the SCANCK 212 and SCANENA 210 signals from tester110, via bus 114. Adaptor 504 outputs SCANCK-A signal 712, SCANCK-Bsignal 714, SCANCK-C signal 716, ENABUF-A signal 718, ENABUF-B signal720, ENABUF-C signal 722, and the SCANENA signal 210. The SCANENA signal210 is connected to all scan cell 200 multiplexers 202 as shown in FIG.2. The SCANCK-A signal 712 is connected, in substitution of SCANCKsignal 212, to all scan cell 200 D-FF 204 clock inputs of scan path A.The SCANCK-B signal 714 is connected, in substitution of SCANCK signal212, to all scan cell 200 D-FF 204 clock inputs of scan path B. TheSCANCK-C signal 716 is connected, in substitution of SCANCK signal 212,to all scan cell 200 D-FF 204 clock inputs of scan path C. The ENABUF-Asignal 718 is connected to the enable input of buffer 512. The ENABUF-Bsignal 720 is connected to the enable input of buffer 514. The ENABUF-Csignal 722 is connected to the enable input of buffer 516.

[0022] Adaptor 504 includes a state machine 702, counter 704, and gates706-710. During functional mode of circuit 500, SCANENA is high asindicated at time 810 in the adaptor timing diagram of FIG. 8. WhileSCANENA is high, state machine 702 outputs control signals 724-728 thatenable SCANCK to pass through gates 706-710 to functionally clock allD-FFs 204 of scan paths A, B, and C, via SCANCK-A, SCANCK-B, andSCANCK-C. In this example, the SCANCK is assumed to be the functionalclock during the functional mode of circuit 500, and the test clockduring test mode of circuit 500. While SCANENA is high, state machine702 outputs control signals 718-722 to disable buffers 512-516. The scanoperation mode is entered by SCANENA going low as indicated at time 812in FIG. 8. SCANENA goes low when tester 110 transitions from the idlestate 302 to the operate state 304 as seen in FIG. 6.

[0023] At the beginning of the scan operation mode, the state machineinitializes counter 704 via control (CTL) signals 730 and disables scanaccess to scan paths B and C by disabling SCANCK gates 708 and 710 viasignals 726 and 728, and enables scan access to scan path A by; (1)enabling SCANCK gate 706 via signal 724, and (2) enabling buffer 512 viasignal 718. Scan access of scan path A occurs over time interval 802 ofFIG. 8. During time interval 802, scan path A is accessed to loadstimulus data from tester 110 via path 118 and unload response to tester110 via path 120. While scan path A is being accessed, the state machineoperates counter 704 via control signals 730 to determine the number(M/3) of SCANCK-A's to output to scan path A. When the counter reaches acount, indicative of scan path A receiving the correct number (M/3)SCANCK-A inputs, it outputs a first count complete 1 (CC1) signal 732 tostate machine 702.

[0024] In response to the first CC1 signal, the state machineinitializes counter 704 via control signals 730 and disables scan accessto scan path A and C, and enables scan access to scan path B over timeinterval 804. The state machine enables scan access to scan path B by;(1) enabling SCANCK gate 708 via signal 726, and (2) enabling buffer 514via signal 720. While scan path B is being accessed, the state machineoperates counter 704 via control signals 730 to determine the number ofSCANCK-B's to output to scan path B. When the counter reaches a count,indicative of scan path B receiving the correct number (M/3) SCANCK-Binputs, it outputs a second count complete 1 (CC1) signal 732 to statemachine 702.

[0025] In response to the second CC1 signal, the state machineinitializes counter 704 via control signals 730 and disables scan accessto scan path A and B, and enables scan access to scan path C over timeinterval 806. The state machine enables scan access to scan path C by;(1) enabling SCANCK gate 710 via signal 728, and (2) enabling buffer 516via signal 722. While scan path C is being accessed, the state machineoperates counter 704 via control signals 730 to determine the number ofSCANCK-C's to output to scan path C. When the counter reaches a count,indicative of scan path C receiving the correct number (M/3) SCANCK-Cinputs, it outputs a third count complete 1 (CC1) signal 732 to statemachine 702.

[0026] In response to the third CC1 signal, the state machine disablesall buffers 512-516 via signals 718-722 and enables gates 706-710 topass the SCANCK to all scan cells of scan paths A, B, and C. Since scanpaths A, B, and C were assumed to contain equal numbers of scan cells(M/3) with the sum of the scan cells in scan paths A, B, and C beingequal to the number of scan cells (M) in scan path 104, the third CC1signal occurs one SCANCK prior to tester 110 setting the SCANENA signalhigh, at time 814, during its transition from the operate state 304 tothe capture state 306 in FIG. 6. While SCANENA is high, at time 808, allscan paths A, B, and C receive a SCANCK, causing them to load responsedata from logic 108 of FIG. 5. Following the response data loadoperation at time 808, SCANENA, from tester 110, returns low at time 812and the above described sequence of separately accessing scan paths A,B, and C repeats until the test completes and tester 110 transitionsback to idle state 302 of FIG. 6.

[0027] Contrasting the scan timing diagrams of FIGS. 4 and 8, it is seenthat tester 110 provides the same SCANENA timing for both diagrams. Forexample, (1) the SCANENA high to low transition at time 406 in FIG. 4 isthe same SCANENA high to low transition at time 812 in FIG. 8, (2) theSCANENA low to high transition at time 408 in FIG. 4 is the same SCANENAlow to high transition at time 814 in FIG. 8, (3) the same number ofSCANCKs occur between time 406/812 and time 408/814 in both diagrams,and (4) the same response load SCANCK occurs at time 404 in FIG. 4 andat time 808 in FIG. 8. The difference between the two timing diagrams isseen in the way the adaptor 504 sequentially applies a burst of M/3SCANCKs to scan paths A, B, and C during time intervals 802, 804, and806, respectively, such that only one of the scan paths is accessed at atime.

[0028] While the example adaptor circuit of FIG. 7 has been describedusing a gated clocking scheme to control access to the scan cells 200 ofscan paths A, B, and C, other example designs of adaptor 504 may be usedto control access to other types of scan cells used in scan paths A, B,and C as well. For example, the scan cells 200 of FIG. 2 could bedesigned to include a state hold multiplexer 218 between the output ofmultiplexer 202 and input to D-FF 204. The state hold multiplexer 218could be controlled, via a connection 220 to the ENACK-A 724, ENACK-B726, and ENACK-C 728 signals from state machine 702, such that itprovides a connection 222 between the output of multiplexer 202 and theD-FF input, or it provides a state hold connection 224 between theoutput of DFF 204 and the input to D-FF 204. If this type of scan cell200 were used in scan paths A, B, and C, the SCANCK 212 could bedirectly routed to all the D-FF 204 clock inputs instead of being gatedto the D-FF 204 clock inputs via the SCANCK-A, SCANCK-B, and SCANCK-Csignals as described for adaptor 504 of FIG. 7. The adaptor 504 would bemodified to operate the state holding scan cells by eliminating thegates 706-710 and the SCANCK-A, SCANCK-B, and SCANCK-C outputs, andproviding as outputs the ENACK-A 724, ENACK-B 726, and ENACK-C 728signals from state machine 702. The ENACK-A output would be connected ascontrol input 220 to the state hold multiplexers 218 in the scan cellsof scan path A. The ENACK-B output would be connected as control input220 to the state hold multiplexers 218 in the scan cells of scan path B.The ENACK-C output would be connected as control input 220 to the statehold multiplexers 218 in the scan cells of scan path C.

[0029] During functional and response capture operations, the ENACK-A,ENACK-B, and ENACK-C outputs from the modified adaptor 504 would be setto enable a connection between the response signal 206 and input to D-FF204 of each scan cell, via multiplexer 202 and the state holdmultiplexer 218. During scan operations to scan path A (timing interval802), the ENACK-B and ENACK-C outputs would be set to place the scancells of scan paths B and C in their state hold connectionconfiguration, and ENACK-A would be set to form a connection between thescan input 208 and input to D-FF 204 of the scan cells in scan paths A,to allow scan access of scan path A. During scan operations to scan pathB (timing interval 804), the ENACK-A and ENACK-C outputs would be set toplace the scan cells of scan paths A and C in their state holdconnection configuration, and ENACK-B would be set to form a connectionbetween the scan input 208 and input to D-FF 204 of the scan cells inscan paths B, to allow scan access of scan path B. During scanoperations to scan path C (timing interval 806), the ENACK-A and ENACK-Boutputs would be set to place the scan cells of scan paths A and B intheir state hold connection configuration, and ENACK-C would be set toform a connection between the scan input 208 and input to D-FF 204 ofthe scan cells in scan paths C, to allow scan access of scan path C.

[0030] The modified adaptor 504 and state hold type scan cells describedabove operate to achieve the low power mode of scan access to scan pathsA, B, and C as previously described with the original adaptor 504 andscan cell 200. The difference between the two adaptor/scan cellcombinations described above is that the original adaptor/scan cellcombination operates in a gated clock mode (i.e. uses gated clocksSCANCK-A, SCANCK-B, and SCANCK-C) and the modified adaptor/scan cellcombination operates in a synchronous clock mode C (i.e. uses theSCANCK).

[0031] Scan Path Adaptation

[0032] As mentioned previously, test synthesis tools exist that arecapable of automatically instantiating scan architectures similar to theone shown in FIG. 1. These tools are capable of analyzing logic 108 andits stimulus and response interface to scan path 104 to determine whatstimulus test pattern data needs to input from tester 110 to logic 108via scan path 104 and what response test patterns data is expected to beoutput to tester 110 from logic 108 via scan path 104. To reduce theeffort required to adapt the synthesized scan architecture of FIG. 1into the low power scan architecture of FIG. 5, the scan path adaptationprocess described below is preferably performed.

[0033] In FIG. 9, scan path 104 is shown receiving stimulus frames 920from tester 110 via connection 118 and outputting response frames 922 totester 110 via connection 120. The term “frame” simply indicates thenumber of scan bits (M) required to fill the scan path 104 with stimulusdata from tester 110 and empty the scan path 104 of response data totester 110 during the operate state 304 of FIG. 3. The test may requirea large number of stimulus and response frame communications to testlogic 108. To achieve the low power mode of operation of the presentinvention, it is desired to reorganize scan path 104 into a plurality ofseparate scan paths. In this example, the reorganization of scan path104 results in the previously described scan path 502, which containsthree separate scan paths 506-510. It is also desired to adapt scan path104 into scan path 502 in such a way as to avoid having to make anymodifications to the stimulus and response test pattern frames 920 and922.

[0034] As previously mentioned in regard to FIG. 5, the number (M) ofscan cells in scan path 104, is assumed divisible by three such thatscan path 104 can be seen to comprise three separate scan segments A, B,and C, each scan segment containing a third (M/3) of the scan cells (M)in scan path 104. Scan segment A of 104 contains a subset 912 of thestimulus and response signals of the overall stimulus and responsebusses 122 and 124 respectively. Scan segment B of 104 contains a subset910 of the stimulus and response signals of the overall stimulus andresponse busses 122 and 124 respectively. Scan segment C of 104 containsa subset 912 of the stimulus and response signals of the overallstimulus and response busses 122 and 124 respectively.

[0035] Each stimulus scan frame 920 scanned into scan path 104 fromtester 110 can be viewed as having bit position fields [CBA] that fillscan segments C, B, and A, respectively. For example, following a scanoperation, bit position field A is loaded into segment A, bit positionfield B is loaded into segment B, and bit position field C is loadedinto segment C. Likewise, each response scan frame 922 scanned from scanpath 104 to tester 110 can be viewed as having bit position fields [CBA]that empty scan segments C, B, and A, respectively. For example,following a scan operation, bit position field A is unloaded fromsegment A, bit position field B is unloaded from segment B, and bitposition field C is unloaded from segment C. To insure that the stimulus920 and response 922 frames are reusable when scan path 104 isreorganized into the low power configuration, the reorganization processoccurs as described below.

[0036] Scan path 104 segment A is configured as a separate scan path A506, as indicated by the dotted line 914. Scan path 104 segment B isconfigured as a separate scan path B 508, as indicated by the dottedline 916. Scan path 104 segment C is configured as a separate scan pathC 510, as indicated by the dotted line 918. The scan inputs to scanpaths A, B, and C 506-510 are connected to tester 110 via connection118. The scan outputs from scan paths A, B, and C 506-510 are connected,via the previously described 3-state buffers 512-516, to tester 110 viaconnection 120. Each separate scan path 506-510 maintains the samestimulus and response bussing connections 908-912 to logic 108.

[0037] Operating the reorganized scan path 502 using the tester 110 usedto operate scan path 104 results in the following behavior. Thisbehavior assumes adaptor 504 has been inserted between the tester 110and scan path 502, to control scan path 502 as described in FIGS. 5, 6,7, and 8. During input and output of stimulus and response frames [CBA]920 and 922 respectively, (1) stimulus bit field A is directly loadedinto scan path A from tester 110 via path 118 as response bit field A isdirectly unloaded from scan path A to tester 110 via path 120, (2)stimulus bit field B is directly loaded into scan path B from tester 110via path 118 as response bit field B is directly unloaded from scan pathB to tester 110 via path 120, and (3) stimulus bit field C is directlyloaded into scan path C from tester 110 via path 118 as response bitfield C is directly unloaded from scan path C to tester 110 via path120. As seen from this description, when scan path 104 is reorganizedinto scan path 502 as described, scan path 502 can use the same stimulusand response frames originally intended for use by scan path 104. Thusno modifications are necessary to the stimulus and response test patternframes produced by the test synthesis tool.

[0038] In the case where scan path 104 contains a number of scan cells(M) that is not equally divisible by the desired number of separate scanpaths (N) in scan path 502, the length of one of the separate scan pathscan be adjusted to compensate scan path 502 for proper input and outputof the scan frames 920-922. For example, if the number of scan cells (M)in scan path 104 is not equally divisible by the number of separate scanpaths (N) required to achieve a desired low power mode of operation, Mcan be increased by adding a value (Y) such that M+Y is equallydivisible by N. Once this is done, N separate scan paths may be formed.N-1 of the separate scan paths will have a length (M+Y)/N and one of theseparate scan paths will have a length of ((M+Y)/N)−Y. For example, ifscan path 104 had 97 scan cells (M), scan path A and B of 502 would eachbe configured to contain 33 scan cells [(M+Y)/N=(97+2)/3=33 ], whilescan path C would be configured to contain 31 scan cells[((M+Y)/N)−Y=((97+2)/3)−2=31 ]. In this example, the scan frame 920-922[CBA] segments would be seen as; segment A=33 bits, segment B=33 bits,and segment C=31 bits.

[0039] When scan path 502 is formed to include the scan framecompensation technique described above, the operation of adaptor 504 isadjusted so it can properly control the compensated scan path 502. InFIGS. 7 and 8, the adaptor 504 circuit and operation was described indetail. Assuming the adaptor timing diagram in FIG. 8 is being used tocommunicate scan frames to a scan path 502 consisting of the abovementioned 33-bit scan path A, 33-bit scan path B, and 31-bit scan pathC, the following changes are required to adaptor 504. Adaptor statemachine 702 continues to monitor the CC1 732 output from counter 704, aspreviously described, to determine when to stop 33-bit scan operationsto scan paths A and B at timing intervals 802 and 804, respectively, inFIG. 8. However, since the scan timing interval 806 to scan path C isdifferent from the scan timing intervals 802 and 804, the state machineoperation is altered to where it monitors the count complete 2 (CC2)output 734 from counter 704 to stop the 31-bit scan operation to scanpath C. The CC2 734 output is designed to indicate when the 31-bit scanoperation to scan path C should be stopped, whereas the CC1 732 isdesigned to indicate when the 33-bit scan operation to scan paths A andB should be stopped.

[0040] Parallel Scan Architectures

[0041]FIG. 10 illustrates circuit 1000 which has been configured fortesting using a conventional parallel scan architecture. As with theprevious single scan architecture of FIG. 1, parallel scan architecturesmay be synthesized and automatically inserted into ICs to serve asembedded testing mechanisms. The parallel scan architecture includesseparate scan paths 1-N 1010-1016 and an interface to tester 1008.During functional mode of circuit 1000, the D-FFs 204 of scan paths 1-Nare configured to operate with logic 1006 to provide the circuit 1000functionality. During test mode, the D-FFs 204 of scan path 1-N areconfigured to operate with tester 1008 to provide testing of logic 1006.Scan paths 1-N receive response from logic 1006 via paths 1040-1046, andoutput stimulus to logic 1006 via paths 1048-1054. Scan paths 1-Nreceive serial stimulus from tester 1008 via paths 1010-1024, and outputserial response to tester 1008 via paths 1026-1032. Scan paths 1-Nreceive control input from tester 1008 via path 1034.

[0042] When circuit 1000 is first placed in the test configuration ofFIG. 10, the parallel scan architecture will be controlled, by tester1008, to be in the idle state 1102 of the test operation diagram 1100 inFIG. 11. From the idle state 1102, tester 1008 will transition theparallel scan architecture into the operate scan paths 1-N state 1104.During the operate state 1104, tester 1008 outputs control to scan paths1-N causing the scan paths to input stimulus from tester 1008 via paths1018-1024 and output response to tester 1008 via paths 1026-1032. Afterthe scan paths 1-N are filled with stimulus and emptied of response,tester 1008 transitions to the capture state 1106 to load the nextresponse data, then returns to the operate state 1104 to input the nextstimulus data and empty the next response data. After all stimulus andresponse data patterns have been applied, by repeating transitionsbetween the operate and capture states, the test is complete and thetester returns to the idle state 1102.

[0043] The structure and operation of the parallel scan architecture ofFIG. 10 is very similar to the structure and operation of the singlescan architecture of FIG. 1. Some of the most notable differencesbetween the scan architectures of FIGS. 1 and 10 include. (1) In FIG.10, multiple parallel scan paths 1-N are formed during the testconfiguration, as opposed to the single scan path 104 formed during theFIG. 1 test configuration. (2) In FIG. 10, tester 1008 outputs multipleparallel stimulus outputs 1018-1024 to scan paths 1-N, as opposed totester 110 outputting a single stimulus output 118 to scan path 104. (3)In FIG. 10, tester 1008 inputs multiple parallel response outputs1026-1032 from scan paths 1-N, as opposed to tester 110 inputting asingle response output 120 from scan path 104.

[0044] The parallel scan architecture of FIG. 10 suffers from the samepower consumption problem described in the scan architecture of FIG. 1,since during scan operations, logic 1006 receives simultaneous ripplingstimulus inputs from scan paths 1-N. Thus, the parallel scanarchitecture of FIG. 10 can be improved to where it consumes less powerduring test by adapting it into a low power parallel scan architectureas described below.

[0045] Low Power Parallel Scan Architecture

[0046]FIG. 12 illustrates the FIG. 10 parallel scan architecture afterit has been adapted for low power operation. The adaptation process, aspreviously described in the low power adaptation of the FIG. 1 scanarchitecture, involves the following steps. Step one includesreconfiguring scan paths 1-N 1010-1016 of FIG. 10 into scan paths 1-N1202-1208 of FIG. 12, wherein each scan path 1-N 1202-1208 containsmultiple separate scan paths between their respective inputs 1018-1024and outputs 1026-1032. In this example, it is assumed that each scanpath 1-N 1202-1208 has been reconfigured into separate scan paths A, B,and C, as scan path 104 of FIG. 1 was reconfigured into scan path 502 ofFIG. 5. Step two includes inserting adaptor 1210 between tester 1008 andscan paths 1-N 1202-1208. In this example, it is assumed that adaptor1210 is very similar to adaptor 504 in the way it operates the separatescan paths A, B, and C in each of the scan paths 1-N 1202-1208, so onlythe brief operation description of adaptor 1210 is given below.

[0047] As seen in the operation diagram of FIG. 13, adaptor 1210responds to tester 1008 entering the operate state 1104 to: (1)simultaneously operate the scan paths A of scan paths 1202-1208, viacontrol bus 1212, to input stimulus from tester 1008 and output responseto tester 1008, then (2) simultaneously operate the scan paths B of scanpaths 1202-1208, via control bus 1212, to input stimulus from tester1008 and output response to tester 1008, then (3) simultaneously operatethe scan paths C of scan paths 1202-1208, via control bus 1212, to inputstimulus from tester 1008 and output response to tester 1008. Adaptor1210 suspends scan operations to scan paths 1202-1208 when tester 1008enters the capture state 1106, and resumes the above described scanoperation sequence to the scan paths A, B, and C of scan paths 1202-1208when tester 1008 re-enters the operate state 1104. After the testcompletes, tester 1008 enters the idle state 1102 and the adaptor 1210is disabled. From this description, the operation of adaptor 1210 isseen to mirror the operation of adaptor 504 with the exception thatadaptor 1210 controls multiple scan paths A, multiple scan paths B, andmultiple scan paths C during its control state diagram sequence 1302. Incontrast, adaptor 504 controlled only one scan path A, one scan path B,and one scan path C during its control state diagram sequence 602.

[0048] Direct Synthesis of Low Power Scan Architectures

[0049] While the process of adapting pre-existing scan architectures forlow power operation has been described, it is anticipated that, once thelow power benefit of the present invention is understood, test synthesistools will be improved to provide direct synthesis of low power scanarchitectures. Direct synthesis of low power scan architectures willeliminate the need to perform the adaptation steps previously described,since the steps will be incorporated into the synthesis process. Adirect synthesis of a single scan path low power scan architecture wouldresult in the direct instantiation of a low power scan architecturesimilar to the one described and shown in regard to FIG. 5. A directsynthesis of a parallel scan path low power scan architecture wouldresult in the direct instantiation of a low power scan architecturesimilar to one described and shown in regard to FIG. 12.

[0050] Adapting Scan Controller Architectures for Low Power Operation

[0051]FIG. 14 illustrates a circuit 1400 configured into a conventionalscan controller based scan architecture. The scan architecture consistsof logic 1410, scan paths 1412-1418, and scan controller 1402. The scanpaths are coupled to logic 1410 via stimulus and response paths 1424, toscan controller 1402 via path 1404, and to tester 1408 via scan inputs1420 and scan outputs 1422. The scan controller is coupled to tester1408 via path 1406. While the scan controller based architecture of FIG.14 uses parallel scan paths 1412-1418, a single scan path architecture,such as the one shown in FIG. 1, could be used as well. The scanarchitecture operates to test logic 1410 as previously described inregard to the scan architecture of FIG. 10, with the exception that thetester 1408 inputs control to scan controller 1402 instead of directlyto the scan paths. In response to tester control input, the scancontroller outputs scan control 1404 to scan paths 1412-1418 to executethe test. An example control diagram for the scan controller is shown inFIG. 15. While various different control diagrams for various differentscan controllers could be shown, the diagram of FIG. 15 reflects thebasic scan operations typically required by any scan controller circuit1402. Those operations being, an idle state 1502, an operate scan state1504, and a capture response state 1506. It is understood that variousother scan operation states could exist in the control diagram.

[0052] The scan controller of FIG. 14 could be anyone of many types ofscan controller circuits. Two examples of some of the types of scancontrollers that could be represented by scan controller 1402 are listedbelow.

[0053] In one realization, scan controller 1402 could represent the testaccess port (TAP) controller circuit of IEEE standard 1149.1, A StandardTest Access Port and Boundary Scan Architecture. A description of theIEEE TAP being used to control scan access to parallel scan paths isdescribed in regard to FIG. 14a of U.S. Pat. No. 5,526,365 by Whetseland is incorporated herein by reference. The TAP operation states differfrom the operation state diagram of FIG. 15, but in general it containsthe fundamental scan 1504 and capture 1506 states.

[0054] In another realization, scan controller 1402 could represent theboundary input/output serializer (BIOS) circuit, described in regard toFIG. 17 of the above mentioned U.S. Pat. No. 5,526,365, being used tocontrol scan access to parallel scan paths. The BIOS description in U.S.Pat. No. 5,526,365 is incorporated herein by reference. The BIOSoperation also differs from the operation state diagram of FIG. 15, butin general it contains the fundamental scan 1504 and capture 1506states.

[0055] In still another realization, scan controller 1402 couldrepresent the addressable test port (ATP) circuit, described in TIpatent application TI-28058, being used to control scan access toparallel scan paths. The TI-28058 patent application is incorporatedherein by reference. As with the TAP and BIOS, the ATP operation differsfrom the operation state diagram of FIG. 15, but in general it containsthe fundamental scan 1504 and capture 1506 states.

[0056]FIG. 16 illustrates the two modification steps to the scancontroller based scan architecture of FIG. 15 to achieve the desired lowpower mode of operation. In the first modification step, as with thepreviously described modification of the FIG. 10 scan architecture intothe FIG. 12 low power scan architecture, each of the scan paths1412-1418 of FIG. 14 are converted into low power scan paths 1602-1608.Each of the low power scan paths 1602-1608 of FIG. 16 contain separatescan path segments A, B, and C arranged as shown and describedpreviously in regard to FIG. 5.

[0057] In the second modification step, an adaptor 1610 is insertedbetween the scan controller 1402 and scan paths 1602-1608. Adaptor 1610inputs control from scan controller 1402 via path 1404 and outputscontrol to scan paths 1602-1608 via path 1612. From the general controlstate diagram example shown in FIG. 17, adaptor 1610 responds to scancontroller 1402 output states (idle state 1502, operate scan paths 1-Nstate 1504, and capture response data state 1506) to output controlstate sequences 1702 (operate scan paths A 1704, operate scan paths B1706, and operate scan paths C 1708) to scan paths 1602-1608. As withprevious adaptor descriptions, the control output from adaptor 1610 toscan paths 1602-1608 operates the scan paths 1602-1608 such that onlyone of the scan path segment groups (i.e. segment group A, B, or C) ofscan paths 1602-1608 are enabled to shift data at a time.

[0058] Since the adaptor's 1610 control input 1404 may come from anytype of scan controller, such as the TAP, BIOS, or ATP mentioned above,the adaptor 1610 design will need to be customized to interface with thespecific scan controller 1402 being used. In general, an adaptor 1610can interface to any given scan controller by simply sensing when thescan controller starts a scan operation and sensing when the scancontroller stops a scan operation. For example, when a scan controllerstarts a scan operation the adaptor starts executing its operate scanpaths A, B, and C state sequence, and when the scan controller stops ascan operation the adaptor stops executing its operate scan paths A, B,and C state sequence.

[0059] While FIG. 16 illustrates the adaptor 1610 as being a circuitseparate from scan controller circuit 1402, the two circuits can bedesigned as one circuit. For example, if it is desired to provide thelow power scan mode of the present invention in a scan controller basedarchitecture, the scan controller and the adaptor circuit functions maybe integrated into a single circuit realization. The previouslymentioned IEEE 1149.1 TAP scan controller may indeed be designed toinclude the adaptor's 1610 scan path segment A, B, and C sequencingfunctionality. Likewise, the previously mentioned BIOS or ATP scancontrollers may indeed be designed to include the adaptor's 1610 scanpath segment A, B, and C sequencing functionality.

[0060] Daisychained Low Power Scan Paths

[0061]FIG. 18 illustrates an IC 1800 having three intellectual propertycore circuits (core1, core2, core3) 1802-1806 configured into adaisychained arrangement for simultaneous parallel scan testing. Cores1-3 could each be a DSP, CPU, or other circuit type. In the scan testconfiguration, each core includes a logic circuit to be tested, and Nscan paths for communicating stimulus and response test patterns to thelogic circuit. The scan paths 1-N of cores 1-3 are assumed to have thesame length. The scan inputs of the core 1 scan paths are connected to atester, such as tester 1008 of FIG. 10, via scan input paths 1808. Thescan outputs of the scan paths of core 1 are connected to the scaninputs of the core 2 scan paths via connections 1814. The scan outputsof the scan paths of core 2 are connected to the scan inputs of the core3 scan paths via connections 1816. The scan outputs of the core 3 scanpaths are connected to a tester, such as tester 1008 of FIG. 10, viascan output paths 1812. The scan paths of cores 1-3 are connected to acontrol bus 1810 to synchronize their daisychained scan test operation.Control bus 1810 could be the control bus 1034 of tester 1008 of FIG.10, or it could be the control bus 1404 of scan controller 1402 of FIG.14.

[0062] During test operation, the scan paths of cores 1-3 are controlledto repeat the steps of; (1) performing a capture operation to loadresponse data from their respective logic circuits, and (2) performing ashift operation to unload response data to the tester via path 1812 andload the next stimulus data from the tester via path 1808. The durationof the shift operation is such that all the daisychained scan paths areemptied of their captured response data and filled with their nextstimulus data. From inspection of FIG. 18 it is seen that during theshift operation, the logic circuits of cores 1-3 receive ripplingstimulus inputs which consumes power in the logic circuits.

[0063]FIG. 19 illustrates an IC 1900 having three intellectual propertycore circuits (core1, core2, core3) 1902-1906 configured into adaisychained arrangement for simultaneous parallel scan testing. Thisexample is provided to demonstrate that cores 1-3 could each have adifferent number of scan paths when configured into the test mode. Forexample, core 1 has two scan paths, core 2 has three scan paths, andcore 3 has N scan paths. Scan paths 1 and 2 of cores 1-3 are seriallyconnected between the tester scan outputs 1914 and 1916 of connection1908 and the tester scan inputs 1922 and 1924 of connection 1912. Scanpaths 3 of core 2 and core 3 are serially connected between the testerscan output 1918 of connection 1908 and the tester scan input 1926 ofconnection 1912. Scan paths 4-N of core 3 are serially connected betweenthe tester scan outputs 1920 of connection 1908 and the tester scaninputs 1928 of connection 1912.

[0064] As in the FIG. 18 example, each core 1-3 of FIG. 19 includes alogic circuit to be tested, and stimulus and response connectionsbetween the logic circuits and the respective scan paths of each core.As in FIG. 18, the scan paths of cores 1-3 are assumed to have the samelength. The scan paths of cores 1-3 are connected to a control bus 1910to synchronize their daisychained scan test operation. As mentioned inregard to FIG. 18, control bus 1910 could come from a tester, such astester 1008 or from a scan controller, such as scan controller 1402.

[0065] During test operation, the scan paths of cores 1-3 are controlledto repeat the steps of; (1) performing a capture operation to loadresponse data from their respective logic circuits, and (2) performing ashift operation to unload response data to the tester via path 1912 andload the next stimulus data from the tester via path 1908. The durationof the shift operation is such that the longest daisychained scan patharrangement (i.e. the daisychain arrangement of scan paths 1 and 2 ofcores 1-3) is emptied of captured response data and filled with nextstimulus data. The scan patterns communicated to the daisychained scanpaths 3 between tester connections 1918 and 1926 will be padded with bitpositions to balance their bit length to the bit length of the scanpatterns communicated to the daisychained scan paths 1 and 2 betweentester connections 1914, 1916, 1922, and 1924. Likewise, the scanpatterns communicated to scan paths 4-N between tester connections 1920and 1928 will be padded with additional bit positions to balance theirlength to the bit length of the scan patterns communicated to thedaisychained scan paths 1 and 2 between connections 1914, 1916, 1922,and 1924. From inspection of FIG. 19 it is seen that during the shiftoperation, the logic circuits of cores-1-3 receive rippling stimulusinputs which consumes power in the logic circuits.

[0066] Adapting Daisychained Scan Paths for Low Power Operation

[0067]FIG. 20 illustrates IC 2000, which is the IC 1800 after the scanpaths 1-N of cores 1-3 of IC 1800 have been converted into low powerscan paths 1-N. Besides the conversion of the conventional scan paths1-N into low power scan paths 1-N, the IC 2000 is the same as that of IC1800. The cores 1-3 of FIG. 20 are the cores 1-3 of FIG. 18. The logiccircuits of the cores 1-3 of FIG. 20 are the logic circuits of cores 1-3of FIG. 18. The scan input connections 2008 of FIG. 20 are the scaninput connections 1808 from the tester of FIG. 18. The scan outputconnections 2012 of FIG. 20 are the scan output connections 1812 to thetester of FIG. 18. The scan path connections 2014 and 2016 of FIG. 20are the scan path connections 1814 and 1816 of FIG. 18. The low powerscan paths of FIG. 20 are assumed to each be partitioned into separatescan path segments A, B, and C, as previously described in regard to thepartitioning of scan path 104 of FIG. 1 into low power scan path 502 ofFIG. 5. The scan paths of cores 1-3 of FIG. 20 are connected to acontrol bus 2010 to synchronize their daisychained scan test operation.The control bus 2010 of FIG. 20 differs from the control bus of FIG. 18in that it comes from an adaptor, such as from adaptor 504 of FIG. 5,adaptor 1210 of FIG. 12, or adaptor 1610 of FIG. 16.

[0068] During test operation, the low power scan paths of cores 1-3 arecontrolled to repeat the steps of; (1) performing a capture operation toload response data from their respective logic circuits, and (2)performing an adaptor controlled shift operation to unload response datato the tester via path 2012 and load the next stimulus data from thetester via path 2008. The adaptor control sequences through the operatescan paths A, B, and C states as previously described in regard to statediagram 1302 of FIG. 13. The duration of the adaptor controlled shiftoperation is such that all the daisychained low power scan paths areemptied of their captured response data and filled with their nextstimulus data. From inspection of FIG. 20 it is seen that during theadaptor controlled shift operation, the logic circuits of cores-1-3receive rippling stimulus inputs only from the currently shifting scanpaths A, B, or C of each low power scan path. Thus the power consumed bythe core 1-3 logic circuits in FIG. 20 during shift operations isreduced from the power consumed by the core 1-3 logic circuits of FIG.18 during shift operations.

[0069]FIG. 21 illustrates IC 2100, which is the IC 1900 after the scanpaths of cores 1-3 of IC 1900 have been converted into low power scanpaths 1-N. Besides the conversion of the conventional scan paths intolow power scan paths, the IC 2100 is the same as that of IC 1900,including the scan path connections to each other and to the tester asdescribed in regard FIGS. 18 and 20 above. The low power scan paths ofFIG. 21 are assumed to each be partitioned into separate scan pathsegments A, B, and C, as previously described in regard to thepartitioning of scan path 104 of FIG. 1 into low power scan path 502 ofFIG. 5. The scan paths of cores 1-3 of FIG. 21 are connected to acontrol bus 2110 to synchronize their daisychained scan test operation.The control bus 2110 of FIG. 21 differs from the control bus of FIG. 19in that it comes from an adaptor, such as from adaptor 504 of FIG. 5,adaptor 1210 of FIG. 12, or adaptor 1610 of FIG. 16.

[0070] During test operation, the low power scan paths of cores 1-3 arecontrolled to repeat the steps of; (1) performing a capture operation toload response data from their respective logic circuits, and (2)performing an adaptor controlled shift operation to unload response datato the tester via path 2112 and load the next stimulus data from thetester via path 2108. The adaptor control sequences through the operatescan paths A, B, and C states as previously described in regard to statediagram 1302 of FIG. 13. The duration of the adaptor controlled shiftoperation is such that the longest daisychained low power scan pathconnection (i.e. the scan path 1 and 2 connections between cores 1-3)are emptied of their captured response data and filled with their nextstimulus data. From inspection of FIG. 21 it is seen that during theadaptor controlled shift operation, the logic circuits of cores-1-3receive rippling stimulus inputs only from the currently shifting scanpaths A, B, or C of each low power scan path. Thus the power consumed bythe core 1-3 logic circuits in FIG. 21 during shift operations isreduced from the power consumed by the core 1-3 logic circuits of FIG.19 during shift operations.

[0071] As previously described in regard to the test times of using scanpaths 104 and 502 to test logic 108 of FIGS. 1 and 5, the test times ofusing the scan paths of FIGS. 18 and 20 to test the logic circuits ofcores 1-3 are the same, as are the test times of using scan paths ofFIGS. 19 and 21 to test the logic circuits of cores 1-3. Also, aspreviously described in regard to FIG. 9, the tester scan input and scanoutput pattern frames used for testing the FIG. 18 cores can be directlyreused to test the FIG. 20 cores. Likewise, the test patterns used fortesting the FIG. 19 cores can be reused to test the FIG. 21 cores.

[0072] As mentioned, the scan paths of the FIG. 18 and 19 circuits wereassumed to be of equal length (M). Also, the corresponding low powerscan paths of FIGS. 20 and 21 were assumed to be modified from the Mlength scan paths FIGS. 18 and 19 such that the scan path segments A, B,and C of each low power scan path are of equal length (M/3). With theseassumptions made, a single adaptor circuit can be used to operate allthe low power scan paths of cores 1-3 of FIGS. 20 and 21 during scanoperations. The single adaptor circuit would sequence through threecycles of scan burst time intervals 802, 804, and 806 of FIG. 8 duringeach scan operation. For example, using the scan operation timingdiagram of FIG. 8 modified as described below, the scan operation of theFIGS. 20 and 21 scan paths can be understood. In FIG. 8, when SCANENAgoes low at time 812 to start the scan operation, a first cycle of scanburst intervals 802-806 occurs, followed by SCANENA remaining low whilea second cycle of scan burst intervals 802-806 occurs, followed bySCANENA remaining low while a third cycle of scan burst intervals802-806 occurs. Following the third cycle of scan burst intervals802-806 SCANENA returns high to end the scan operation.

[0073] While the adaptor may be designed to operate the low power scanpaths different from the operation described above, the above describedoperation maintains the ability to reuse the test pattern frames of theoriginal scan paths of FIG. 18 and 19. For example, each of the existingscan test pattern frames (stimulus and response) for the FIGS. 18 and 19circuit can be viewed in the format of “[core1] [core2] [core3]”, where[corel] indicates the scan frame bit positions targeted for the core 1scan paths, [core2] indicates the scan frame bit positions targeted forthe core 2 scan paths, and [core3] indicates the scan frame bitpositions targeted for the core 3 scan paths. It follows from theprevious description given for FIG. 9 that; [corel] can be furtherviewed in a format of [C₁B₁A₁], [core2] can be further viewed in aformat of [C₂B₂A₂], and [core3] can be further viewed in a format of[C₃B₃A₃], where the C_(1,2,3), B_(1,2,3), and A_(1,2,3) subset bitpositions are targeted for each low power scan path segment C, B, and Aof cores 1-3 of FIG. 22 and 21. The above described adaptor scanoperation would operate to load and unload each subset C_(1,2,3),B_(1,2,3), A_(1,2,3) scan frame bit positions into the respective C, B,and A scan path segments of each low power scan path of FIGS. 20 and 21.The advantage to this, as mentioned in regard to FIG. 9, is that testpattern frames originally provided for the scan path arrangement ofFIGS. 18 and 19 do not have to modified for use with the low power scanpath arrangement of FIGS. 20 and 21.

[0074] As mentioned in regard to the FIG. 19 daisychain arrangement,length compensating pad bit positions are included in the test patternframes communicated to the daisychained scan paths 3 of cores 2 and 3,and in the test pattern frames communicated to scan paths 4-N of core 3.During the adaptor operated scan operation of the FIG. 21 circuit, thesepad bit positions are communicated during the above mentioned cycles ofscan burst timing intervals 802-806, such that at the end of eachadaptor controlled scan operation, all scan path segments A,B,C of alllow power scan paths properly filled with stimulus and emptied ofresponse.

[0075] In FIG. 20, if the A,B,C segments of the low power scan paths ofcore 1 have the same length, the A,B,C segments of the low power scanpaths of core 2 have the same length, and the A,B,C segments of the lowpower scan paths of core 3 have the same length, but the lengths of theA,B,C segments of the low power scan paths of cores 1-3 are not thesame, separate adaptor interfaces will be needed to control the lowpower scan paths of each core 13. A first adaptor interface will beconnected to the control input 2018 of core 1 to provide control of thecore 1 A,B,C segments, a second adaptor interface will be connected tothe control input 2020 of core 2 to provide control of the core 2 A,B,Csegments, and a third adaptor interface will be connected to the controlinput 2022 of core 3 to provide control of the core 3 A,B,C segments.The use of separate adaptor interfaces allows each of the core 1-3 lowpower scan paths to be operated according to the scan burst timingintervals (802-806) required to communicate to each of the differentlength low power scan path A,B,C segments of cores 1-3. For example, ifthe A,B,C segment lengths of cores 1, 2, and 3 were 100, 300, and 900respectively, the scan burst intervals (802-806) of core 1 would be setat 100 each, the scan burst timing intervals (802-806) of core 2 wouldbe set at 300 each, and the burst timing intervals (802-806) of core 3would be set at 900 each. A single adaptor circuit may be equipped withmultiple separate interfaces for connection to control inputs 2018-2022,or separate adaptor circuits may be interfaced to control inputs2018-2022. In a daisychained arrangement, as in FIG. 20, each core mayhave different A,B,C scan segment lengths. However for proper daisychainoperation, each of the different core A,B,C scan lengths should be setto positive integer multiples of one another such that the adpatorinterfaces to each core can operate together during each scan operationcycle (i.e. from SCANENA going low at 812 to SCANENA going high at 814in FIG. 8) to modulate the test patterns through all daisychained coreswithout loosing any of the stimulus and response pattern bits. Forexample, the 100, 300, and 900 A,B,C core scan segment lengths mentionedabove have been set to where, during each scan operation cycle, theadaptor of core 1 modulates test patterns through core 1 using multiplecycles of 100 bit scan burst intervals (802-806), the adaptor of core 2modulates test patterns through core 2 using multiple cycles of 300 bitscan burst intervals (802-806), and the adaptor of core 3 modulates testpatterns through core 3 using multiple cycles of 900 bit scan burstintervals (802-806). Since 900 is a multiple of 300 and 300 is amultiple of 100, all A,B,C scan paths of cores 1-3 will be properlyfilled and emptied during each scan operation cycle.

[0076] Scalable Scan Architecture Power Consumption

[0077] As can be anticipated from the description given for the presentinvention, the power consumption of logic circuitry being tested by thelow power scan architecture decreases as the number separate scan pathswithin the low power scan paths increases. For example, configuring agiven conventional scan path into a low power scan path comprising twoseparate scan paths may reduce power consumption by up to 50%, since,during operation, each of the two separate scan paths separately chargeand discharge one half, potentially, of the logic circuitry capacitancecharged and discharged by the convention scan path. Further, configuringthe same conventional scan path into a low power scan path comprisingthree separate scan paths may reduce power consumption by up to 66%,since, during operation, each of the three separate scan pathsseparately charge and discharge one third, potentially, of the logiccapacitance charged and discharged by the convention scan path. Stillfurther, configuring the same conventional scan path into a low powerscan path comprising four separate scan paths may reduce powerconsumption by up to 75%, since, during operation, each of the fourseparate scan paths separately charge and discharge one fourth,potentially, the logic capacitance charged and discharged by theconvention scan path. From this it is seen that the present inventionallows a synthesis tool to be provided with the capability of scalingthe power consumption of a given synthesized scan architecture to meet adesired low power mode of test operation of a circuit.

[0078] Scalable Scan Architecture Noise Reduction

[0079] As can be anticipated from the description given for the presentinvention, the noise generated by logic circuitry being tested by thelow power scan architecture decreases as the number separate scan pathswithin the low power scan paths increases. For example, configuring agiven conventional scan path into a low power scan path comprising twoseparate scan paths may reduce noise generation by up to 50%, since,during operation, each of the two separate scan paths separatelyactivate only one half, potentially, of the logic circuitry activated bythe conventional scan path. Further, configuring the same conventionalscan path into a low power scan path comprising three separate scanpaths may reduce noise generation by up to 66%, since, during operation,each of the three separate scan paths separately activate only onethird, potentially, of the logic circuitry activated by the conventionscan path. Still further, configuring the same conventional scan pathinto a low power scan path comprising four separate scan paths mayreduce noise generation by up to 75%, since, during operation, each ofthe four separate scan paths separately activate one fourth,potentially, of the logic circuitry activated by the convention scanpath. From this it is seen that the present invention allows a synthesistool to be provided with the capability of scaling the noise generationof a given synthesized scan architecture to meet a desired low noisemode of test operation of a circuit.

[0080] Circuit 220 of FIG. 22 illustrates an alternative method ofcontrolling low power scan segments A 506, B 508, and C 510 during test.Circuit 2200 is similar to circuit 500 of FIG. 5 with the followingexceptions; (1) adaptor 504 has been replaced with a decode logiccircuit 2201, (2) additional externally accessible control inputs C12202 and C0 2203 have been provided and connected to the decode logic2201, and (3) the tester 2204 has been equipped with outputs forcontrolling the C1 and C0 inputs.

[0081]FIG. 23 illustrates the low power scan operation mode 2300 ofcircuit 2200 when being controlled by decode logic 2201 via the scaninterface 114 and C1 and C0 inputs from tester 2204. At the beginning ofthe low power scan test, tester 2204 inputs scan control (SCANENA andSCANCK) and a 0:0 code on C1 and C0 to cause the circuit to transitionfrom the idle state 2301 (functional mode) to the operate scan path Astate 2302. In operation state 2302, scan path A 506 and its outputbuffer 512 are enabled by bus 518 from decode logic 2201 to shiftstimulus data in and response data out via paths 118 and 120,respectively. When scan path A has been filled with stimulus and emptiedof response, the tester inputs a 0:1 code on C1 and C0, respectively,which causes entry into operation state 2303. In operation state 2303,scan path B 508 and its output buffer 514 are enabled by bus 520 fromdecode logic 2201 to shift stimulus data in and response data out viapaths 118 and 120, respectively. When scan path B has been filled withstimulus and emptied of response, the tester inputs a 1:0 code on C1 andC0, respectively, which causes entry into operation state 2304. Inoperation state 2304, scan path C 510 and its output buffer 516 areenabled by bus 522 from decode logic 2201 to shift stimulus data in andresponse data out via paths 118 and 120, respectively. When scan path Chas been filled with stimulus and emptied of response, the tester inputsa 1:1 code on C1 and C0, respectively, which causes entry into operationstate 2305. In response to the 1:1 code, decode logic 2201 outputscontrol on buses 518, 520, and 522 to disable all output buffers 512-516and cause all scan paths A, B, C 506-510 to capture response data fromlogic 108. The above described sequence of the tester is repeated forall required scan pattern cycles by the tester appropriately re-enteringthe C1 and C0 code sequences of 0:0, 0:1, 1:0, and 1:1 . At the end oftest, the test returns circuit 2200 to its functional mode byre-entering the idle state.

[0082] Example Decode Logic Circuit

[0083]FIG. 24 illustrates an example decode logic circuit 2201implementation. Decode logic 2201 inputs the SCANCK 212 and SCANENA 210signals from tester 2204, via bus 114. Decode logic 2201 outputsSCANCK-A signal 712, SCANCK-B signal 714, SCANCK-C signal 716, ENABUF-Asignal 718, ENABUF-B signal 720, ENABUF-C signal 722, and the SCANENAsignal 210. The SCANENA signal 210 is connected to all scan cell 200multiplexers 202 as shown in FIG. 2. The SCANCK-A signal 712 isconnected, in substitution of SCANCK signal 212, to all scan cell 200D-FF 204 clock inputs of scan path A. The SCANCK-B signal 714 isconnected, in substitution of SCANCK signal 212, to all scan cell 200D-FF 204 clock inputs of scan path B. The SCANCK-C signal 716 isconnected, in substitution of SCANCK signal 212, to all scan cell 200D-FF 204 clock inputs of scan path C. The ENABUF-A signal 718 isconnected to the enable input of buffer 512. The ENABUF-B signal 720 isconnected to the enable input of buffer 514. The ENABUF-C signal 722 isconnected to the enable input of buffer 516. While 3-state buffers512-516 are shown providing selectable connectivity between the outputsof scan paths A, B, and C and bus 120, other switching circuits, such asmultiplexers, could be used as well.

[0084] Decode logic 2201 includes combinational decode circuit 2401 andgates 706-710. Decode logic 2201 inputs the C1 and C0 signals andoutputs the scan clock enable signals 724-728 and buffer enable signals718-722. During functional mode of circuit 2200, SCANENA is high asindicated at time 2501 in the timing diagram of FIG. 25. Also duringfunctional mode of circuit 2200 at time 2501, the C1 and C0 signals areboth high. While C1 and C0 are high, decode circuit 2401 outputs controlsignals 724-728 that enable SCANCK to pass through gates 706-710 tofunctionally clock all D-FFs 204 of scan paths A, B, and C, viaSCANCK-A, SCANCK-B, and SCANCK-C. In this example, the SCANCK is assumedto be the functional clock during the functional mode of circuit 2200,and the test clock during test mode of circuit 2200. Also while C1 andC0 are high, decode circuit 2401 outputs control signals 718-722 todisable buffers 512-516. The scan operation mode is entered by tester2204 inputting a low on SCANENA and a 0:0 code (i.e. lows) on C1 and C0,respectively, as indicated at time 2502 in FIG. 25.

[0085] Decode circuit 2401 responds to the 0:0 code on C1 and C0 to setENACK-A 724 and ENABUF-A to a state that enables SCANCK-A 712 and buffer512. While C1:C0 =0:0 , a burst 2503 of SCANCK-A's is output from decodelogic 2201 to shift data through scan path A from path 118 to 120. Whenscan path A has been shifted, tester 2204 inputs a 0:1 code on C1 and C0respectively. Decode circuit 2401 responds to the 0:1 code on C1 and C0to set ENACK-B 726 and ENABUF-B to a state that enables SCANCK-B 714 andbuffer 514. While C1:C0 =0:1, a burst 2504 of SCANCK-B's is output fromdecode logic 2201 to shift data through scan path B from path 118 to120. When scan path B has been shifted, tester 2204 inputs a 1:0 code onC1 and C0 respectively. Decode circuit 2401 responds to the 1:0 code onC1 and C0 to set ENACK-C 728 and ENABUF-C to a state that enablesSCANCK-C 716 and buffer 516. While C1:C0=1:0 , a burst 2505 ofSCANCK-C's is output from decode logic 2201 to shift data through scanpath C from path 118 to 120.

[0086] When scan path C has been shifted, tester 2204 inputs a high onSCANENA and a 1:1 code on C1 and C0. Decode circuit 2401 responds to the1:1 code on C1 and C0 to set ENACK-A, ENACK-B, and ENACK-C 724-728 tostates that enable SCANCK-A, SCANCK-B, and SCANCK-C 712-716 to operatesimultaneously, and to set ENABUF-A, ENABUF-B, and ENABUF-C to statesthat disable buffers 512-516. While SCANENA is high and C1:C0 =1:1 attime 2506, scan paths A, B, and C operate to capture response data fromlogic 108 during each SCANCK-A, B, and C clock. While the timing diagramof FIG. 25, illustrates two response capture operations occurring duringtime 2506, any number of response capture operations could be performed.At the end of the response capture operation, the tester 2204 repeatsthe above described input sequence on SCANENA and C1 and C0 to apply allremaining test patterns to logic 108.

[0087] Contrasting the decode logic 2201 controlled timing of FIG. 25with the adaptor 504 controlled timing of FIG. 8, it is seen that thelow power scan test operation is similar in both. The primary differenceis that the decode logic 2201 approach of FIGS. 22-25 require additionalinputs (i.e. the C1 and C0 inputs) from tester 2204 to control when scanpaths A, B, and C are operated, whereas the adaptor 504 approach ofFIGS. 5-8 does not require the additional inputs from tester 110.

[0088] The decode logic 2201 of FIG. 24 can be easily described to anautomatic test pattern generation tool, enabling the tool to easilygenerate the scan test patterns to be used by tester 2204. Since tester2204 can directly control the C1 and C0 inputs to select which scan pathA, B, or C is selected for scanning, debug of a failing device, i.e. afailing IC or sub-circuit within an IC, is facilitated. For example, ifa device test fails and it is desired to determine why the failureoccurred, it is possible for the tester to directly and individuallyselect one of the scan paths, say scan path A 512, so that a moreexhaustive scan test can be applied to the portion of the logiccircuitry 108 associated with the individually selected scan path A.Likewise, scan paths B and C can be directly and individually selectedto execute more exhaustive scan tests on the portions of the logiccircuitry 108 associated with the scan paths B and C. Furthermore, andby manipulation of the C1 and C0 inputs, it is possible for tester 2204to efficiently execute the steps of; (1) individually selecting andscanning a stimulus pattern into scan paths A, B, and C for input tologic 108, (2) performing one or more capture operations as shown attime 2506 of FIG. 25 to load scan paths A, B, and C with response fromlogic 108, and (3) individually selecting and scanning the capturedresponse patterns from scan paths A, B, and C.

[0089] Since the tester 2204 controls the C1 and C0 inputs, the scanpaths A, B, C may be set to any desired bit length. For example, in FIG.25, shift event 2503 may be set to 100 bits in length, shift event 2504may be set to 200 bits in length, and shift event 2505 may be set to 300bits in length. The different shift event bit lengths are easilycontrolled by the C1 and C0 inputs from the tester. However, while C1and C0 provide flexible control over shift event lengths, optimum scanpower reduction is achieved only when the shift event lengths are madeequal or near equal, as previously mentioned in regard to FIGS. 1 and 5.Making the shift lengths equal or near equal decreases the number ofscan cells 220 that are simultaneously accessed during each shift event,which reduces the number of rippling stimulus inputs to logic 108. Thusfor reducing power during scan test and regardless of whether scancontrol comes from adaptor 504 of FIG. 5 or the decode logic 2201 ofFIG. 22, it is desirable to partition a conventional scan path up intolow power scan path segments that have equal or near equal bit lengths.

[0090] In FIG. 24, a SCANENA signal 2402 is shown being output fromdecode circuit 2401 and connected, via dotted line, to the SCANENAsignal 210. This is shown to simply indicate that, if desired, theSCANENA signal 210 can be produced by decode of the C1 and C0 inputs. Ifdecode circuit 2401 provides an internally produced SCANENA signal 2402and connects the signal to SCANENA 210, the tester does not need toprovide an externally produced SCANENA control input to the device viabus 114 of FIG. 22. Using the timing diagram of FIG. 25, a 1:1 code onC1 and C0 would set the SCANENA 2402 signal high at times 2501 and 2506,while all other codes (i.e. 0:0, 0:1, 1:0 ) would set the SCANENA 2402signal low at times 2503, 2504, and 2505 respectively. The low powerscan operation depicted in FIG. 25 using an internally produced SCANENA2402 signal to control the SCANENA 210 input to multiplexers 202 of FIG.2 is identical to using an externally provided SCANENA signal on bus 114from a tester. The benefit of using an internally produced SCANENAsignal 2401 is that it allows for reducing the control input bus 114from tester 2204 by one input.

[0091] In FIG. 25, it is seen that two external control inputs (C0-C1)can be decoded into three individual scan path shift events 2503-2505and one capture event 2506. Thus two external control inputs providecontrol for up to three individual low power scan paths A,B,C. If morethan three low power scan paths are used, the number of control inputswould need to be increased to decode the additional shift events andcapture event. For example, three control inputs (C0-C2) would be neededfor a low power scan path arrangement that included seven separate shiftevents and one capture event.

[0092]FIGS. 26 and 27 are provided to illustrate that a single decodelogic 2201 and set of C1 and C0 inputs can be used to access a parallelarrangement of low power scan paths 2601-2604, similar to the way asingle adaptor 504 was illustrated doing so in FIGS. 12 and 13. Itshould be clear that decode logic 2201 in combination with the C1 and C0control inputs could be substituted for adaptor 504 in all low powerscan examples described herein.

[0093] It should be clear that it is possible to eliminate decode logic2201 altogether if enough external control inputs are provided to allowtester 2204 to directly control the enabling of scan clocks 712-716 toscan paths A, B, and C via gates 706-710 and the switching of the scanpath A, B, and C serial outputs onto bus 120 via buffer 512-516. Forexample, in FIG. 24 the tester could provide separate control inputs forthe scan clock gating signals 724-728 and the scan path serial outputswitching signals 718-722. Furthermore, it is possible to provideexternal control inputs that would allow direct and separate control ofscan clocks 712-716, if desired. This expansion of control inputs wouldprovide greater flexibility in the operation and control of low powerscan path arrangements.

[0094] Although the present invention has been described in accordanceto the embodiments shown in the figures, one of ordinary skill in theart will recognize there could be variations to these embodiments andthose variations should be within the spirit and scope of the presentinvention. Accordingly, modifications may be made by one ordinarilyskilled in the art without departing from the spirit and scope of theappended claims.

What is claimed is: 1 A scan test configuration of a circuit within anintegrated circuit, said scan test configuration comprising, multiplescan paths each having a scan input terminal, scan output terminal, ascan enable input terminal, and a scan clock input terminal, decodelogic having externally accessible control input terminals and separatescan clock output terminals, a connection formed between the scan inputterminals, a connection formed between the scan enable input terminals,and connections formed between ones of the separate scan clock outputterminals and said scan clock input terminals such that each separatescan clock output terminal is connected to only one of said scan clockinput terminals. 2 An integrated circuit comprising; a scan path havinga scan input, a scan output, a scan clock input, and a scan enableinput, test control input terminals on the integrated ciruit;combinational logic within the integrated circuit for inputting saidtest control inputs and for outputting, in response to said controlinputs, a scan enable output, and a connection formed between said scanenable output and said scan enable input. 3 A scan test configuration ofa circuit within an integrated circuit, said scan test configurationcomprising, multiple scan paths each having a scan input terminal, scanoutput terminal, a scan enable input terminal, and a scan clock inputterminal, decode logic having externally accessible control inputterminals, at least one scan clock output terminal, and at least onescan enable output terminal, a connection formed between said at leastone scan enable output terminal and said scan enable input terminals,and a connection formed between said at least one scan clock outputterminal and said scan clock input terminals. 4 A scan testconfiguration of a circuit within an integrated circuit, said scan testconfiguration comprising, multiple scan paths each having a scan inputterminal, scan output terminal, a scan enable input terminal, and a scanclock input terminal, decode logic having externally accessible controlinput terminals, scan clock output terminals, and a scan enable outputterminal, a connection formed between said scan enable output terminaland said scan enable input terminals, and separate connections formedbetween each of said scan clock output terminals and one of said scanclock input terminals. 5 A scan test configuration of a circuit withinan integrated circuit, said scan test configuration comprising, multiplescan paths each having a scan input terminal, scan output terminal, ascan enable input terminal, and a scan clock input terminal, externallyprovided control input terminals, a connection formed between the scaninput terminals, a connection formed between at least one of saidexternally provided control input terminals and the scan enable inputterminals, and connections formed between other ones of said externallyprovided control input terminals and said scan clock input terminalssuch that each scan clock input terminal may be operated separate fromthe other scan clock input terminals.